In this paper experimental results of investigation of the oscillations in a photosynthetic system are presented and a model for their interpretation is suggested. Periodicities in photosynthetic systems detected in earlier studies by physical chemical methods can be also detected by means of recording the potential difference between two point electrodes. The observed dependences demonstrate a wide range of various types of behaviour of the system, working, e.g. in periodic, quasiperiodic, chaotic or 'pulse' regimes. Since the until-now-used 2-dimensional theoretical model, based on the existence of two dominant autocatalytical processes, appeared not to be sufficient for explaining such types of the regimes, a generalized 3-dimensional autocatalytical model is suggested, which is able to explain all the above mentioned photosynthetic regimes.